Mouse mammary tumorigenesis is a multistep process involving at least two major events: the transformation of normal cells to preneoplastic cells and the transformation of preneoplastic cells to tumor cells. Markers capable of detecting cells in different stages of the preneoplastic state would be valuable tools for dissecting and defining the underlying sequential events in neoplastic progression in mammary epithelium. The most thoroughly studied preneoplastic lesion is the hyperplastic alveolar nodule (HAN). We have recently prepared antibodies against 3 cytoskeletal components that may represent such markers in mouse mammary epithelial cells: a 46kd keratin, recognized by a monospecific antiserum, that is found in some HAN and all mouse mammary carcinomas in vivo; a perinuclear structure (PNS), visualized by a rat monoclonal antibody designated PNS-Ab, that is present in cells of all mammary tumors and at least one type of HAN; and an apparent keratin epitope, detected by a rat monoclonal antibody designated MKE- Ab, that is found in all normal and some HAN cells but not in tumor cells. The purpose of the proposed research is to analyze the relationship of expression of these 3 components to mammary tumorigenesis more thoroughly and to determine if the presence of any of these components can be used in identifying cells in different steps of the progression of HAN to malignancy. The antibodies will be used in immunocytochemistry in three types of experiments involving mouse mammary tissues: 1) to follow the emergence of cells expressing the 46kd keratin and the PNS while losing the MKE-Ab determinant during the development of primary HAN and tumors in mammary tumorigenesis of BALB/cfC3H mice; 2) to examine cells of HAN with high and low tumor potentials for expression of the 3 components at different times during the latency period of tumor development and determine if the appearance of any of them correlates with tumor incidence; and 3) to investigate the effect of increasing the tumor potential of an HAN on expression of the 3 components. Additional experiments will determine if treatment of normal mouse mammary epithelial cells in culture with a carcinogen, e.g. DMBA, results in the appearance of cells expressing the PNS and if similar treatment of HAN cells induces a loss of the MKE-Ab determinant. If such cells are found, they will be tested for altered growth and histogenic potential in vivo by transplantation experiments.